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This invention relates to airborne antennas and, more particularly, to such antennas providing multiple beam excitation usable with anti-jam adaptive processing to suppress jamming and interference.
A variety of antennas have been made available for reception of Global Positioning System (GPS) signals for navigational and other purposes. A more critical objective than the mere capability to receive such signals, is the objective of enabling reception in the presence of interference or jamming signals. Interference may be the unintended result of reception of signals radiated for some purpose unrelated to GPS operations. Jamming, on the other hand, may involve signals intentionally transmitted for the purpose of obstructing reception of GPS signals. In airborne operations which are dependent upon use of GPS signals, deleterious effects of interference or jamming may be particularly disruptive.
For reception via a fixed-position antenna in the presence of interference signals incident from a fixed azimuth, for example, a reduced-gain antenna pattern notch aligned to suppress reception at the appropriate azimuth may be employed as an effective solution. However, for airborne operations a more complex solution is required. With an aircraft and its antenna operable in a variety of geographical locations and conditions, with constantly changing azimuth orientation during flight, interference or jamming signals may be incident from any azimuth and with constantly changing azimuth. At the same time, maneuvers such as banked turns of an aircraft, for example, tilt the aircraft and its antenna so that the interference or jamming signals may be incident from different and changing elevation angles.
A variety of adaptive processing techniques have previously been described. Such techniques typically provide an anti-jam capability based on provision of reduced-gain antenna pattern notches and alignment of such notches at the incident azimuth of undesired incoming signals. However, to enable practical employment of such techniques for reception of GPS signals under critical airborne operations, reliable, low-profile antennas providing a multi-beam capability suitable for anti-jam application are required.
Examples of prior antennas meeting most of these objectives include those provided in U.S. patent application Ser. No. 09/789,467, filed Feb. 21, 2001, and having a common assignee with the present application. That application describes, in particular, a GPS antenna including four bent monopoles in combination with four slot elements to provide primary and auxiliary antenna patterns usable for aircraft anti-jam applications.
Airborne applications may include large aircraft, smaller fighter and drone aircraft where small antenna size is important, and smaller objects such as missiles, guided bombs and other projectiles. In the latter categories of applications size, weight, cost and complexity become increasingly important, along with antenna anti-jam operational capabilities. Prior types of GPS antennas have typically not fully met overall objectives of small size and low weight, cost and complexity, with concurrent high performance and multiple auxiliary antenna patterns usable for anti-jam adaptive processing for such applications.
Accordingly, objects of the present invention are to provide new and improved aircraft antennas having one or more of the following characteristics and capabilities:
low-profile configuration of eight monopole elements in circular array;
eight elements with eight beam excitation capability;
omnidirectional circularly-polarized principal beam;
seven selectively usable auxiliary beams;
full hemispherical beam coverage;
multiple elements for omnidirectional and other coverage;
small-size, low-profile implementation;
high-performance, high-reliability design;
usable in a variety of beam configurations for anti-jam applications; and
multiple pattern excitation suitable for adaptive processing anti-jam operation.
In accordance with the invention, an eight-element GPS antenna, usable with multi-pattern adaptive processing for anti jam operation includes a ground plane portion, eight monopole elements positioned above the ground plane portion and an excitation network coupled to the monopole elements. The excitation network is configured to provide output signals representative of each of the following antenna patterns;
(i) 45 degree counter-clockwise (CCW) progressive phase excitation of the monopole elements to produce a first circularly-polarized omnidirectional antenna pattern;
(ii) 45 degree clockwise (CW) progressive phase excitation of the monopole elements to produce a second circularly polarized omnidirectional antenna pattern;
(iii) 90 degree CCW progressive phase (PP) excitation of the monopole elements to produce a 90 degree CCW PP antenna pattern;
(iv) 90 degree CW progressive phase excitation of the monopole elements to produce a 90 degree CW PP antenna pattern;
(v) 135 degree CCW progressive phase excitation of the monopole elements to produce a 135 degree CCW PP antenna pattern;
(vi) 135 degree CW progressive phase excitation of the monopole elements to produce a 135 degree CW PP antenna pattern;
(vii) 180 degree progressive phase excitation of the monopole elements to produce an eight-lobe antenna pattern; and
(viii) same phase excitation of the monopole elements to produce a uniform phase omnidirectional antenna pattern.
In other embodiments, antennas may be arranged to utilize only some of the above antenna patterns in different selected combinations and may include other patterns.
For a better understanding of the invention, together with other and further objects, reference is made to the accompanying drawings and the scope of the invention will be pointed out in the accompanying claims.